Because the synthetic technology of 1,2-propanediol is relatively mature, and the quality and output of product are relatively stable, therefore, utilize propylene glycol to be the report of main raw material synthesis propylene carbonate more.
1) Propylene glycol-phosgene method: The earliest industrial preparation of propylene carbonate was the synthesis reaction of 1,2-propanediol and phosgene.
Because phosgene is a highly toxic substance, it causes serious harm to people and the environment; in addition, the generation of by-product hydrochloric acid not only reduces the atomic economy of the process, but also increases the process investment cost due to the corrosion of equipment by hydrochloric acid. Therefore, the law is currently prohibited.
2) Propylene glycol-urea method
The route of urea and 1,2-propanediol to synthesize propylene carbonate has been studied more in China. When propylene glycol reacts with urea to synthesize propylene carbonate, the first step is to generate aminocarbonate, and the second step is deamination and cyclization of aminocarbonate to generate the target product propylene carbonate, accompanied by the generation of by-product ammonia. The reaction conditions introduced in the patent of preparing propylene carbonate with urea and propylene glycol reported earlier are mild, and the yield of the target product is relatively high. The catalysts introduced are organotins, which are somewhat toxic.
Switching to a solid base catalyst can reduce the toxicity of the process. In the presence of a solid base, such as zinc oxide, the reaction temperature is 100-200 ° C, nitrogen gas is introduced, and after a certain period of time, the yield of propylene carbonate calculated by urea can reach 99%. When the composite calcium oxide catalyst is used, under reduced pressure, the temperature is 150-160 ℃, the conversion rate of urea is 95%-98%, the selectivity of propylene carbonate is 90%-98%, and the catalyst can be recycled .
Propylene carbonate was synthesized from urea and propylene glycol with basic magnesium carbonate calcined MgO as a catalyst. The yield of PC was greater than 90% after 3 hours of reaction at a reaction temperature of 170 °C. Using inorganic lead and zinc compounds as heterogeneous catalysts, the reaction temperature is 160 ℃, the reaction time is 6 h, the yield of propylene carbonate is 98% based on urea; the reaction product and the catalyst are easily separated. Using iron-zinc oxide as catalyst, the yield of propylene carbonate was 78% after reaction at 170 ℃ for 2 h. The main active component of the catalyst is ZnO, and the catalytic activity is promoted by the joint action of ZnO and ZnFe2O4. The cost of propylene carbonate synthesized by propylene glycol-urea method is relatively low, and it has certain process raw material advantages.
3) Propylene glycol-carbon dioxide method
Carbon dioxide is used in this reaction process. Carbon dioxide is a greenhouse gas. Due to human activities, the concentration of carbon dioxide on the earth’s surface has increased. Using carbon dioxide as a raw material and fixing it into chemicals is a green synthesis idea, and practical reports have been seen. Although the carbon dioxide used in most current studies does not come directly from emissions, the idea is also considered to be green. The catalysts used in this method are alkali metal salts or alkaline earth metal salts, among which potassium carbonate has high catalytic activity. In a homogeneous catalytic system, the yield of propylene carbonate can reach 12.6%.
In order to overcome the difficulties of product separation and catalyst recycling caused by homogeneous catalytic reactions, potassium carbonate was loaded on activated carbon for heterogeneous catalytic reactions. The results showed that the selectivity of products was improved. Solvent acetonitrile was used in the synthetic process of this method, which reduced the degree of greening of its process. Organic tin compounds, such as Bu2SnO or Bu2Sn(OMe)2, can also catalyze the reaction of 1,2-propanediol and carbon dioxide under supercritical conditions to generate propylene carbonate.
The addition of a co-solvent or the presence of a dehydrating agent is beneficial to the formation of the product and the improvement of the yield. During the reaction of 1,2-propanediol and carbon dioxide, water is generated, which reduces the utilization rate of atoms in the reaction process, and the product will be hydrolyzed, so the yield of the product will be inhibited by water. This is a big problem to be solved in the process of realizing the industrialization of the process.
4) Exchange method of propylene glycol and ester
The preparation of propylene carbonate can be accomplished by transesterification with 1,2-propanediol and diethyl carbonate or dimethyl carbonate.
Using alkali metal or alkaline earth metal as a catalyst, reacting at 144 ℃ for 12 h under normal pressure, the yield of propylene carbonate is 88%. If dibutyl tin dilaurate and a small amount of strong base are used as catalysts for the transesterification reaction, xylene is used to reflux to control the reaction temperature, and the by-product ethanol is continuously fractionated, so that the operation steps can be reduced. However, because the raw materials used in this method are relatively expensive, and the catalyst organotin is relatively toxic, it is not an ideal greening process.